1. Field of the Invention
This invention relates to ignition systems for spark-ignited, internal combustion engines which are capable of high pulse repetition rates.
2. Description of the Prior Art
In a spark-ignition internal combustion engine, a flyback transformer is commonly used to generate the high voltage needed to create an arc across the gap of the spark plug and cause an ignition event, i.e. igniting the fuel and air mixture within the engine cylinder. The timing of this ignition spark event is critical for best fuel economy and low exhaust emission of environmentally hazardous gases. A spark event which is too late leads to loss of engine power and efficiency. Correct spark timing is dependent on engine speed and load. Each cylinder of an engine often requires different timing for optimum performance. Different spark timing for each cylinder can be obtained by providing a spark ignition transformer for each spark plug.
To improve engine efficiency and alleviate some of the problems associated with inappropriate ignition spark timing, some engines have been equipped with microprocessor-controlled systems which include sensors for engine speed, intake air temperature and pressure, engine temperature, exhaust gas oxygen content, and sensors to detect xe2x80x9cpingxe2x80x9d or xe2x80x9cknockxe2x80x9d.
Advanced, spark ignited, two and four-stroke engines used in the automotive and related industries may employ an ignition and spark plug system capable of multiple firings during each cylinder ignition stroke. Multiple sparking is known as a means for engine diagnostics.
A disproportionately greater amount of exhaust emission of hazardous gases is created during the initial operation of a cold engine and during idle and off-idle operation. Studies have shown that rapid multi-sparking of the spark plug for each ignition event during these two regimes of engine operation may reduce hazardous exhaust emissions. Accordingly, it is desirable to have a fast cycling spark ignition system.
Engine misfiring increases hazardous exhaust emissions. Numerous cold starts without adequate heat in the spark plug insulator in the combustion chamber can lead to misfires, due to deposits of soot on the insulator. The electrically conductive soot reduces the voltage increase available for a spark event. A spark ignition transformer which provides an extremely rapid rise in voltage can minimize the misfires due to soot fouling.
A coil-per-spark plug (CPP) ignition arrangement in which the spark ignition transformer is mounted directly to the spark plug terminal, eliminating a high voltage wire between the conventional engine coil and spark plug, is gaining acceptance as a method for improving the spark ignition timing of internal combustion engines. One example of a CPP ignition arrangement is disclosed in U.S. Pat. No. 4,846,129 to Noble (hereinafter xe2x80x9cthe Noble patentxe2x80x9d). The physical diameter of the spark ignition transformer must fit into the same engine tube in which the spark plug is mounted. To achieve the engine diagnostic goals envisioned in the Noble patent, the patentee discloses an indirect method utilizing a ferrite core. Ideally the magnetic performance of the spark ignition transformer is sufficient throughout the engine operation to sense the sparking condition in the combustion chamber.
To achieve the spark ignition performance needed for successful operation of the ignition and engine diagnostic system disclosed by Noble and, at the same time, reduce the incidence of engine misfire due to spark plug soot fouling, the spark ignition transformer""s core material: (i) must have moderately high magnetic permeability; and (ii) must have low magnetic losses. In order to achieve critical performance requirements such as very fast rise times and rapid energy transfer, the magnetic core material must be capable of high frequency response with low loss. The combination of these required properties and performance criteria narrows the availability of suitable core materials. Possible candidates for the core material include silicon steel, ferrite, and iron-based amorphous metal. Conventional silicon steel routinely used in utility transformer cores is inexpensive, but its magnetic losses are too high. Thinner gauge silicon steel with lower magnetic losses is too costly. Ferrites are inexpensive, but their saturation inductions are normally less than 0.5 Tesla (T) and their Curie temperatures (at which the core""s magnetic induction becomes close to zero) are typically close to 200xc2x0 C. This temperature is too low considering that the spark ignition transformer""s upper operating temperature is typically about 180xc2x0 C. Iron-based amorphous metal has low magnetic loss and high saturation induction exceeding 1.5 T, however it shows relatively high permeability, limiting its energy storage capability. An iron-based amorphous metal capable of achieving a level of magnetic permeability suitable for a spark ignition transformer is needed. Using this material, it is possible to construct a toroidal coil which meets required output specifications and physical dimension criteria. The dimensional requirements of the spark plug well region limit the type of configurations that can be used. Typical dimensional requirements for plug-mounted, insulated coil assemblies are less than 25 mm in diameter and less than 150 mm in length. These coil assemblies must also attach to the spark plug on both the high voltage terminal and outer ground connection and provide sufficient insulation to prevent arc-over from the coil to other engine components. The outer ground connection can be made via a return from the engine block, as in typical coil-per-plug systems. There must also be the ability to make high current connections to the primary coil windings typically located on top of the coil.
The present invention provides a spark ignition system for an internal combustion engine. The system includes a magnetic core-coil assembly and associated driver electronics and is capable of high pulse rate operation because of its rapid charge time (for example, xcx9c100 microseconds using a 12 volt source), rapid voltage rise (for example, 200-500 nanoseconds), and rapid discharge time (for example, xcx9c150 microseconds). It has low output impedance (30-100 ohms), produces high ( greater than 25 kV) open circuit voltages, and delivers high peak current through the spark (0.4-1.5 ampere) and high spark energy, typically 6-12 millijoules per pulse. Operation from a 12 volt battery source is readily accomplished using simple driver electronics at rates ranging from single shot to about 4 kHz, which are considerably greater than the current ignition systems can offer. The core-coil assembly may actually be operated using any voltage  greater than 5 volts to supply the driver electronics input voltage. The upper voltage supply limit is dependent on the voltage rating of the components used within the driver electronics, so the present system may be operated with conventional 12 V power or with readily available components at higher supply voltages including the 40-50 Volt system now being contemplated within the automotive industry. The charging time of the core-coil assembly is related to the supply voltage of the driver electronics. The higher the supply voltage, the faster the current will increase through the primary winding of the core-coil. This is due to loss reduction in the components that comprise the driver electronics and the ability to source more current. At lower voltages, the voltage drop across the switching element of the driver electronics (typically an IGBT) will limit the available voltage drop across the core-coil. This has the effect of increasing the charge time until a pre-determined current is flowing through the core-coil primary. This type of electronic system (electronic driver plus core-coil) output delivered through a surface gap plug (typical of avionic spark ignition systems) or a conventional J gap spark plug or derivatives results in a high power ignition source with localized heating capability. A xe2x80x9cspark plugxe2x80x9d or alternative term xe2x80x9cignitorxe2x80x9d refers to a device that requires high voltage to create a spark across a gap. That gap can be a ceramic which is typical of a surface gap ignitor, or it can be an air gap, which is typical of a xe2x80x9cJxe2x80x9d gap spark plug. A xe2x80x9cJxe2x80x9d gap derivative refers to any other type of spark plug where an arc must be created over a distance similar to the distance between electrodes of a conventional xe2x80x9cJxe2x80x9d gap spark plug. The magnetic core-coil assembly and ignition system of the invention may be operated at much higher pulse rates than prior art systems. High pulse rates, such as the 4 kHz or more the present system can provide, have a number of advantages applicable to reciprocating engines. Typical ignition systems used in automotive applications are limited to pulse rates of about 110 Hz by their long charge and discharge times. In spark-ignited engines a high pulse rate allows multiple sparks to be produced during each ignition stroke in either a two or four stroke engine. In this case, spark initiation is generally made synchronous with respect to crankshaft position using known timing signal means. The high cycling rate further allows this synchronization to be more precise.
Generally stated, the magnetic core-coil assembly of the present invention includes a magnetic core comprising at least one tape-wound toroid of ferromagnetic amorphous metal alloy with low magnetic losses and moderately high magnetic permeability. The core-coil assembly has a low-voltage primary coil energized from the driver electronics and a secondary coil for a high voltage output.
A number of core forms are possible, including both a single core with a single primary and a single secondary and a multiple core form. The latter is especially useful in making cylindrical cores with long aspect ratios (i.e., the ratio of length to diameter). A core with a long aspect ratio is known to those in the art as a pencil coil and will be referred to as such hereafter in this disclosure. This assembly has a secondary coil comprising a plurality of core sub-assemblies that are simultaneously energized via the common primary coil for a time during which current flows in the primary, storing energy in a magnetic field within the core material. The core sub-assemblies are adapted, when energized by the driver electronics, to produce secondary voltages. That is to say, during the period that the sub-assemblies are energized by the driver electronics, the primary current is rapidly interrupted, causing the magnetic field within the cores to collapse. Secondary voltages are thereby induced across the each of the secondary windings. These secondary voltages are additive in the pencil coil design, and the voltage is fed to the spark plug via the secondary connection to the spark plug or ignitor.
The single core-coil embodiment has a single primary and a single secondary but operates similarly. Energy is stored in the magnetic core as a result of current flowing through the primary. When the primary current flow is rapidly interrupted by the driver electronics, the magnetic field within the core collapses. A voltage is thereby induced and appears across the single secondary, which is connected to the spark plug or ignitor.
Compared to cores made with prior art materials, cores of the invention made with ferromagnetic amorphous metal alloy require fewer primary and secondary windings due to the magnetic permeability of the core material and exhibit lower magnetic losses. As thus constructed, the core-coil assembly has the capability of generating a high voltage in the secondary coil within a short period of time following excitation thereof.
More specifically, the core of the core-coil assembly is composed of an amorphous ferromagnetic material which exhibits low core loss and a permeability (ranging from about 100 to 500). Such magnetic properties are especially suited for rapid firing of the spark plug during a combustion cycle. Misfires of the engine due to soot fouling are minimized. Moreover, energy transfer from coil to plug is carried out in a highly efficient manner, with the result that very little energy remains within the core after discharge. The low secondary resistance of the generally toroidal core design (typically, less than 100 ohms) permits the bulk of the energy to be dissipated in the spark and not in the secondary winding of the core-coil assembly. In the segmented form the individual secondary voltages generated across the plural core-coil sub-assemblies rapidly increase and add sub-assembly to sub-assembly based on the total magnetic flux change of the system. This allows the versatility to combine several core-coil sub-assemblies wound via existing toroidal coil winding techniques to produce a single assembly with superior performance. This embodiment is especially advantageous for core-coil assemblies with a long aspect ratio, making them less expensive to construct and more efficient and reliable in operation than core-coil assemblies of similar geometry having a single elongated secondary core.
Another embodiment uses a single larger toroidally wound core-coil that produces output characteristics similar to those of the pencil coil (multiple stack arrangement of smaller core-coil assemblies) described above. The unit operates in the manner described above. Use of a single core may be attractive for designs in which a short aspect ratio assembly is acceptable. Only a single core is needed, leading to a simpler manufacture and the resistance of the windings typically lower for a given core cross-sectional area.
The driver electronics comprise a power source (typically a battery), a low Equivalent Series Resistance (ESR) capacitor to supply high peak current, a switch such as an Integrated gate bipolar transistor (IGBT) which can be turned on (shorted condition) to allow current to flow through the coil primary establishing the magnetomotive force and then subsequently turned off (open condition) which rapidly decreases the current flow through the primary of the coil causing the magnetic field to collapse in the core inducing voltage onto the secondary winding producing an output. A timing means may be required to turn the switch on and off at the appropriate times.